This invention relates generally to tri-level highlight color printing and, more particularly, to developer bias switching between a standard DC bias and a chopped DC bias for enabling tri-level highlight color and bi-level black imaging utilizing the same development system for each.
The invention can be utilized in the art of xerography or in the printing arts in the practice of conventional xerography, it is the general procedure to form electrostatic latent images on a xerographic surface by first uniformly charging a photoconductive insulating surface or photoreceptor. The charge is selectively dissipated in accordance with a pattern of activating radiation corresponding to original images. The selective dissipation of the charge leaves a latent charge pattern on the imaging surface corresponding to the areas not struck by radiation.
This charge pattern is made visible by developing it with toner. The toner is generally a colored powder which adheres to the charge pattern by electrostatic attraction.
The developed image is then fixed to the imaging surface or is transferred to a receiving substrate such as plain paper to which it is fixed by suitable fusing techniques.
The concept of tri-level xerography is described in U.S. Pat. No. 4,078,929 issued in the name of Gundlach. The patent to Gundlach teaches the use of tri-level xerography as a means to achieve single-pass highlight color imaging. As disclosed therein, the charge pattern is developed with toner particles of first and second colors. The toner particles of one of the colors are positively charged and the toner particles of the other color are negatively charged. In one embodiment, the toner particles are supplied by a developer which comprises a mixture of triboelectrically relatively positive and relatively negative carrier beads. The carrier beads support, respectively, the relatively negative and relatively positive toner particles. Such a developer is generally supplied to the charge pattern by cascading it across the imaging surface supporting the charge pattern. In another embodiment, the toner particles are presented to the charge pattern by a pair of magnetic brushes. Each brush supplies a toner of one color and one charge. In yet another embodiment, the development system is biased to about the background voltage. Such biasing results in a developed image of improved color sharpness.
In tri-level xerography, the xerographic contrast on the charge retentive surface or photoreceptor is divided three, rather than two, ways as is the case in conventional xerography. The photoreceptor is charged, typically to 900 v. It is exposed imagewise, such that one image corresponding to charged image areas (which are subsequently developed by charged area development, i.e. CAD) stays at the full photoreceptor potential (V.sub.ddp or V.sub.cad, see FIGS. 1a and 1b). The other image is exposed to discharge the photoreceptor to its residual potential, i.e. V.sub.c or V.sub.dad (typically 100 v) which corresponds to discharged area images that are subsequently developed by discharged-area development (DAD). The background areas exposed such as to reduce the photoreceptor potential to halfway between the V.sub.cad and V.sub.dad potentials, (typically 500 ) and is referred to as V.sub.w or V.sub.white. The CAD developer is typically biased about 100 v closer to V.sub.cad than V.sub.white (about 600 v), and the DAD developer system is biased about 100 v closer to V.sub.dad than V.sub.white (about 400 v).
Because the composite image developed on the charge retentive surface consists of both positive and negative toner a pre-transfer corona charging step is necessary to bring all the toner to a common polarity so it can be transferred using corona charge of the opposite polarity.
Various techniques have heretofore been employed to develop electrostatic images as illustrated by the following disclosures which may be relevant to certain aspects of the present invention.
U.S. Pat. No. 4,761,668 granted to Parker et al and assigned to the same assignee as the instant application which relates to tri-level printing discloses apparatus for minimizing the contamination of one dry toner or developer by another dry toner or developer used for rendering visible latent electrostatic images formed on a charge retentive surface such as a photoconductive imaging member. The apparatus causes the otherwise contaminating dry toner or developer to be attracted to the charge retentive surface in its inter-document and outboard areas. The dry toner or developer so attracted is subsequently removed from the imaging member at the cleaning station.
U.S. Pat. No. 4,761,672 granted to Parker et al and assigned to the same assignee as the instant application which relates to tri-level printing disclosed apparatus wherein undesirable transient development conditions that occur during start-up and shut-down in a tri-level xerographic system when the developer biases are either actuated or deactuated are obviated by using a control strategy that relies on the exposure system to generate a spatial voltage ramp on the photoreceptor during machine start-up and shut-down. Furthermore, the development systems' bias supplies are programmed so that their bias voltages follow the photoreceptor voltage ramp at some predetermined offset voltage. This offset is chosen so that the cleaning field between any development roll and the photoreceptor is always within reasonable limits. As an alternative to synchronizing the exposure and developing characteristics, the charging of the photoreceptor can be varied in accordance with the change of developer bias voltage.
U.S. Pat. No. 4,811,046 granted to Jerome E. May and assigned to the same assignee as the instant application which relates to tri-level printing discloses apparatus wherein undesirable transient development conditions that occur during start-up and shut-down in a tri-level xerographic system when the developer biases are either actuated or deactuated are obviated by the provision of developer apparatuses having rolls which are adapted to be rotated in a predetermined direction for preventing developer contact with the imaging surface during periods of start-up and shut-down. The developer rolls of a selected developer housing or housings can be rotated in the contact-prevention direction to permit use of the tri-level system to be utilized as a single color system or for the purpose of agitating developer in only one of the housings at a time to insure internal triboelectric equilibrium of the developer in that housing.
U.S. Pat. No. 4,771,314 granted to Parker et al and assigned to the same assignee as the instant application which relates to tri-level printing discloses printing apparatus for forming toner images in black and at least one highlighting color in a single pass of a charge retentive imaging surface through the processing areas, including a development station, of the printing apparatus. The development station includes a pair of developer housings each of which has supported therein a pair of magnetic brush development rolls which are electrically biased to provide electrostatic development and cleaning fields between the charge retentive surface and the developer rolls. The rolls are biased such that the development fields between the first rolls in each housing and the charge retentive surface are greater than those between the charge retentive surface and the second rolls and such that the cleaning fields between the second rolls in each housing and the charge retentive surface are greater than those between the charge retentive surface and the first rolls.
U.S. Pat. No. 4,833,504 granted to Parker and assigned to the same assignee as the instant application which relates to tri-level printing discloses a magnetic brush developer apparatus comprising a plurality of developer housings each including a plurality of magnetic rolls associated therewith. The magnetic rolls disposed in a second developer housing are constructed such that the radial component of the magnetic force field produces a magnetically free development zone intermediate a charge retentive surface and the magnetic rolls. The developer is moved through the zone magnetically unconstrained and, therefor, subjects the image developed by the first developer housing to minimal disturbance. Also, the developer is transported from one magnetic roll to the next. This apparatus provides an efficient means for developing the complementary half of a tri-level latent image while at the same time allowing the already developed first half to pass through the second housing with minimum image disturbance.
U.S. patent application Ser. No. 220,408 filed on Jun. 28, 1988 in the name of Parker et al and assigned to the same assignee as the instant application which relates to tri-level printing disclosed an electronic printer employing tri-level xerography to superimpose two images with perfect registration during the single pass of a charge retentive member past the processing stations of the printer. One part of the composite image is formed using Magnetic Ink Character Recognition (MICR) toner, while the other part of the image is printed with less expensive black, or color toner. For example, the magnetically readable information on a check is printed with MICR toner and the rest of the check in color or in black toner that is not magnetically readable.
The problem of fringe field development in a tri-level highlight color, single pass imaging system is addressed in U.S. Pat. No. 4,847,655 assigned to the same assignee as the instant invention and granted to Parker et al on Jul. 11, 1989. In this application there is disclosed a magnetic brush developer apparatus comprising a plurality of developer housings each including a plurality of magnetic brush rolls associated therewith. Conductive magnetic brush (CMB) developer is provided in each of the developer housings. The CMB developer is used to develop electronically formed images. The developer conductivity, as measured in a powder electrical conductivity cell, is in the range of 10.sup.-9 to 10.sup.-13 (ohm-cm)-1. The toner concentration of the developer is in the order of 2.0 to 3.0% by weight and the toner charge level is less than 20 microcoulombs/gram and the developer rolls are spaced from the charge retentive surface a distance in the order of 0.40 to 0.120 inch.
U.S. Pat. No. 4,868,611 granted on Sep. 9, 1989 to Richard P. Germain and assigned to the same assignee as the instant invention discloses a highlight color imaging method and apparatus including structure for forming a single polarity charge pattern having at least three different voltage levels on a charge retentive surface wherein two of the voltage levels correspond to two image areas and the third voltage level corresponds to a background area. Interaction between developer materials contained in a developer housing and an already developed image in one of the two image areas is minimized by the use of a scorotron to neutralize the charge on the already developed image.
In high speed (i.e. 135 cpm) xerographic printing excellent copy quality can be obtained by the use of a hybrid development system as disclosed in U.S. Pat. No. 4,537,494, granted to A. Lubinsky et al on Aug. 27, 1985. The '494 patent disclosed the use of a somewhat insulative developer material. When attempting to do highlight color utilizing tri-level xerography in a high speed printer, the development system disclosed in the '494 patent becomes undesirable due to its insulative nature. With an insulative development system, higher development fields are needed to obtain the same developed mass/area (DMA) as would be needed with a conductive system. It also tends to develop fringe fields.
Tri-level xerography requires the development of two images within the same voltage space that is normally used for one image in standard bi-level xerography. As a result, the effective development and cleaning fields available in tri-level imaging are about half that of normal xerography. These lower fields make it more difficult to develop enough toner on the photoreceptor latent image in order to obtain acceptable output densities on paper, while still maintaining acceptable background suppression. While tri-level xerography can achieve sufficient development of both colors with acceptable background, the reduced operating latitudes (as compared to bi-level monochrome xerography) require that process parameters such as Toner Concentration (TC) and photoreceptor electrostatics be carefully controlled, and that the available voltage space of the photoreceptor be maximized (resulting in lower photoreceptor life).
A conductive development system is preferably used in tri-level imaging so that higher DMA's (developed mass/area) for a given background level can be achieved with these lower development fields. The conductive material also suppresses fringe field development which can cause black development around the edges of a color image or visa versa.
U.S. patent application Ser. No. 07/772,306 assigned to the same assignee as the instant application and filed in the USPTO in the name of Germain et al on the same day discloses the use of Chopped DC biases applied to developer structures containing Conductive Magnetic Brush (CMB) developer for extending the operating latitude of tri-level highlight color imaging.